More people are using digital assistants like the Amazon Echo or Alexa.
By one estimate, there are more than 110 million devices in this category in the U.S.
Our interactions with those devices can shape the advertising we see elsewhere.
According to a study published in April, Amazon Echo interactions were used to help target ads on Amazon Prime, Amazon.com, and several other platforms.
"Imagine you are tracking your pregnancy on a health skill, and you ask questions about it. There is a chance that those interactions might be used for targeted advertising related to pregnancy," according to Umar Iqbal, a postdoctoral scholar at the University of Washington who is one of the co-authors of the study.
The researchers created a series of personas based on certain interest profiles, like fitness, religion or politics.
Each persona interacted with an Echo device.
Over time, researchers found the advertising became more tailored to the persona.
"I'm not literally or explicitly saying that I'm interested in fashion," said Pouneh Nikkhah Bahrami, a researcher at the University of California, Davis and a co-author on the study. "But my questions are related to fashion. So the device can process the data and then extract my interests. It figures out that this person is interested in smart devices or fashion styles or a car."
In an emailed statement, an Amazon spokesperson said it's true that some interactions are used for ad targeting.
"For example, if you ask Alexa to order paper towels or to play a particular song on Amazon Music, the record of that purchase or song play may inform relevant ads shown on Amazon or other sites where Amazon places ads," an Amazon spokesperson said in a written statement. "This is not an atypical practice."
However, the company spokesperson clarified that Amazon does not "sell customers' personal information" and does not "share Alexa requests with advertising networks."
"Customers can opt out of interest-based ads from Amazon at any time on our Advertising Preferences page," the spokesperson said. "This includes ads that Amazon serves on our devices and services like Alexa."
Researchers say the company can do more to ensure transparency among third-party skills for its smart speakers.